1. Field of the Invention
The present invention relates to a process for continuously producing a solution of titanium tetrachloride and aluminum trichloride which when evaporated produces a vapor mixture containing a desired particular amount of aluminum trichloride for use in producing rutile titanium dioxide pigment.
2. Description of the Prior Art
The production of rutile titanium dioxide pigment by the oxidation of titanium tetrachloride admixed with a minor amount of aluminum trioxide is well known. The vapor mixture is generally combined with oxygen in a burner wherein the oxidation of the titanium tetrachloride to titanium dioxide takes place. It is necessary to incorporate a minor amount of aluminum trichloride vapor with the titanium tetrachloride vapor to insure the desired rutile crystalline form of titanium dioxide is produced and to improve the weatherability of the titanium dioxide product.
In order to obtain a rutile titanium dioxide of uniform quality, it is necessary to maintain a constant ratio of aluminum compound and titanium tetrachloride in the vapor mixture which is oxidized. A variety of techniques have been developed and used heretofore for producing such vapor mixtures. Examples of the techniques include adding metallic aluminum powder to a mixture of vaporous titanium tetrachloride and oxygen, spraying molten aluminum into a mixture of vaporous titanium tetrachloride and oxygen, passing liquid titanium tetrachloride together with oxygen and an inert gas through solid aluminum trichloride in an evaporator, passing a carrier gas stream of vaporous titanium tetrachloride and oxygen through a fluidized bed of aluminum trichloride, evaporating titanium tetrachloride and aluminum trichloride separately and then combining them in the desired ratio, passing vaporized titanium tetrachloride and chlorine through a bed of aluminum metal particles or a fluidized bed of aluminum powder, and many others.
U.S. Pat. No. 2,824,050 issued Feb. 18, 1958 discloses a process wherein aluminum trichloride is dissolved in hot liquid titanium tetrachloride and the solution is evaporated after removal of non-volatile precipitates produced. The process includes dissolving 0.1% to 5% of the aluminum trichloride in the titanium tetrachloride, distilling the mixture, condensing and combining the distillate, and continuously flash vaporizing the distillate.
Even though a variety of the foregoing and other prior art techniques have been utilized successfully, all have involved the use of relatively expensive equipment, and generally, they have not been consistent in producing a constant ratio of titanium tetrachloride to aluminum compound.
U.S. Pat. No. 3,524,818 issued Aug. 18, 1970 discloses a method which is stated to be relatively simple, inexpensive and continuous for producing a stream of vaporous titanium tetrachloride containing an exact amount of vaporous aluminum trichloride. The method comprises the steps of dissolving the aluminum trichloride in a suitable anhydrous organic or inorganic liquid, and simultaneously feeding a stream of the resulting solution and a separate stream of liquid titanium tetrachloride to an evaporator in controlled amounts whereby both streams are evaporated simultaneously. Examples of solvents which can be used are ethers, alcohols and thionylchloride. Thus, the method involves the use of a solvent which does not enter into the titanium dioxide production reaction, which adds expense to the method and which presents a solvent recovery or disposal problem.
In more recent processes for producing rutile titanium dioxide pigment, aluminum trichloride is dissolved in hot titanium tetrachloride, and the resulting solution is fed to vaporizers which simultaneously vaporize both the aluminum trichloride and titanium tetrachloride to provide a vapor mixture having the desired particular aluminum trichloride content. The vapor mixture is then oxidized in an oxidizer apparatus to produce the rutile titanium dioxide pigment. The vaporizer and oxidizer apparatus must be operated at super atmospheric pressures which makes it difficult to pump the hot highly corrosive titanium tetrachloride-aluminum trichloride solution into the vaporizer apparatus. In order to avoid the pumping difficulties, the vaporizer apparatus has often been fed from an elevated feed tank, and the overall process of producing the titanium tetrachloride-aluminum trichloride solution has still been expensive to carry out. Thus, there is a need for an improved less expensive process for producing a continuous stream of titanium tetrachloride solution containing a desired particular amount of aluminum trichloride which can be vaporized and then utilized as a feed to titanium dioxide producing oxidizer apparatus.